This invention relates to compounds, especially peptides, that have the ability to stimulate in vitro and in vivo-production of platelets and their precursor cells, e.g., megakaryocytes. The following is provided as background regarding two proteins that are known to have thrombopoietic activity: thrombopoietin (TPO) and megakaryocyte growth and development factor (MGDF).
The cloning of endogenous thrombopoietin (TPO) (Lok et al., Nature 369:568-571 (1994); Bartley et al., Cell 77:1117-1124 (1994); Kuter et al., Proc. Natl. Acad. Sci. USA 91:11104-11108 (1994); de Sauvage et al., Nature 369:533-538 (1994); Kato et al., Journal of Biochemistry 119:229-236 (1995); Chang et al., Journal of Biological Chemistry 270:511-514 (1995)) has rapidly increased our understanding of megakaryopoiesis (megakaryocyte production) and thrombopoiesis (platelet production).
Endogenous human TPO, a 60 to 70 kDa glycosylated protein primarily produced in the liver and kidney, consists of 332 amino acids (Bartley et al., Cell 77:1117-1124 (1994); Chang et al., Journal of Biological Chemistry 270:511-514 (1995)). The protein is highly conserved between different species, and has 23% homology with human erythropoietin (Gurney et al., Blood 85:981-988 (1995)) in the amino terminus (amino acids 1 to 172) (Bartley et al., Cell 77:1117-1124 (1994)). Endogenous TPO has been shown to possess all of the characteristics of the key biological regulator of thrombopoiesis. Its in vitro actions include specific induction of megakaryocyte colonies from both purified murine hematopoietic stem cells (Zeigler et al., Blood 84:4045-4052 (1994)) and human CD34+ cells (Lok et al., Nature 369:568-571 (1994); Rasko et al., Stem Cells 15:33-42 (1997)), the generation of megakaryocytes with increased ploidy (Broudy et al., Blood 85:402-413 (1995)), and the induction of terminal megakaryocyte maturation and platelet production (Zeigler et al., Blood 84:4045-4052 (1994); Choi et al., Blood 85:402-413 (1995)). Conversely, synthetic antisense oligodeoxynucleotides to the TPO receptor (c-mpl) significantly inhibit the colony-forming ability of megakaryocyte progenitors (Methia et al., Blood 82:1395-1401 (1993)). Moreover, c-mpl knock-out mice are severely thrombocytopenic and deficient in megakaryocytes (Alexander et al., Blood 87:2162-2170 (1996)).
Recombinant human MGDF (rHuMGDF, Amgen Inc., Thousand Oaks, Calif.) is another thrombopoietic polypeptide related to TPO. It is produced using E. coli transformed with a plasmid containing cDNA encoding a truncated protein encompassing the amino-terminal receptor-binding domain of human TPO (Ulich et al., Blood 86:971-976 (1995)). The polypeptide is extracted, refolded, and purified, and a poly[ethylene glycol] (PEG) moiety is covalently attached to the amino terminus. The resulting molecule is referred to herein as PEG-rHuMGDF or MGDF for short.
Various studies using animal models (Ulich, T. R. et al., Blood 86:971-976 (1995); Hokom, M. M. et al., Blood 86:4486-4492 (1995)) have clearly demonstrated the therapeutic efficacies of TPO and MGDF in bone marrow transplantation and in the treatment of thrombocytopenia, a condition that often results from chemotherapy or radiation therapy. Preliminary data in humans have confirmed the utility of MGDF in elevating platelet counts in various settings. (Basser et al., Lancet 348:1279-81 (1996); Kato et al., Journal of Biochemistry 119:229-236 (1995); Ulich et al., Blood 86:971-976 (1995)). MGDF might be used to enhance the platelet donation process, since administration of MGDF increases circulating platelet counts to about three-fold the original value in healthy platelet donors.
TPO and MGDF exert their action through binding to the c-mpl receptor which is expressed primarily on the surface of certain hematopoietic cells, such as megakaryocytes, platelets, CD34+ cells and primitive progenitor cells (Debili, N. et al., Blood 85:391-401 (1995); de Sauvage, F. J. et al, Nature 369:533-538 (1994); Bartley, T. D., et al., Cell 77:1117-1124 (1994); Lok, S. et al., Nature 369: 565-8 (1994)). Like most receptors for interleukins and protein hormones, c-mpl belongs to the class I cytokine receptor superfamily (Vigon, I. et al., Proc. Natl. Acad. Sci. USA 89:5640-5644 (1992)). Activation of this class of receptors involves ligand-binding induced receptor homodimerization which in turn triggers the cascade of signal transducing events.
In general, the interaction of a protein ligand with its receptor often takes place at a relatively large interface. However, as demonstrated in the case of human growth hormone bound to its receptor, only a few key residues at the interface actually contribute to most of the binding energy (Clackson, T. et al., Science 267:383-386 (1995)). This and the fact that the bulk of the remaining protein ligand serves only to display the binding epitopes in the right topology makes it possible to find active ligands of much smaller size. Accordingly, molecules of only “peptide” length (e.g., 2 to 80 amino acids) can bind to the receptor protein of a given large protein ligand. Such peptides may mimic the bioactitivy of the large protein ligand or, through competitive binding, inhibit the bioactivity of the large protein ligand, and are commonly referred to as “peptide mimetics” or “mimetic peptides.”
Phage display peptide libraries have emerged as a powerful technique in identifying such peptide mimetics. See, e.g., Scott, J. K. et al., Science 249:386 (1990); Devlin, J. J. et al., Science 249:404 (1990); U.S. Pat. No. 5,223,409, issued Jun. 29, 1993; U.S. Pat. No. 5,733,731, issued Mar. 31, 1998; U.S. Pat. No. 5,498,530, issued Mar. 12, 1996; U.S. Pat. No. 5,432,018, issued Jul. 11, 1995; U.S. Pat. No. 5,338,665, issued Aug. 16, 1994; U.S. Pat. No. 5,922,545, issued Jul. 13, 1999; WO 96/40987, published Dec. 19, 1996; and WO 98/15833, published Apr. 16, 1998 (each of which is incorporated by reference in its entirety). In such libraries, random peptide sequences are displayed by fusion with coat proteins of filamentous phage. Typically, the displayed peptides are affinity-eluted against an antibody-immobilized extracellular domain of a receptor. The retained phages may be enriched by successive rounds of affinity purification and repropagation. The best binding peptides may be sequenced to identify key residues within one or more structurally related families of peptides. See, e.g., Cwirla, et al. (1997), Science 276: 1696-9. The peptide sequences may also suggest which residues may be safely replaced by alanine scanning or by mutagenesis at the DNA level. Mutagenesis libraries may be created and screened to further optimize the sequence of the best binders. Lowman (1997), Ann. Rev. Biophys. Biomol. Struct. 26: 401-24.
Structural analysis of protein-protein interaction may also be used to suggest peptides that mimic the binding activity of large protein ligands. In such an analysis, the crystal structure may suggest the identity and relative orientation of critical residues of the large protein ligand, from which a peptide may be designed. See, e.g., Takasaki, et al. (1997), Nature Biotech, 15: 1266-70. These analytical methods may also be used to investigate the interaction between a receptor protein and peptides selected by phage display, which may suggest further modification of the peptides to increase binding affinity.
Other methods compete with phage display in peptide research. A peptide library can be fused to the carboxyl terminus of the lac repressor and expressed in E. coli. Another E. coli-based method allows display on the cell's outer membrane by fusion with a peptidoglycan-associated lipoprotein (PAL). Hereinafter, these and related methods are collectively referred to as “E. coli display.” In another method, translation of random RNA is halted prior to ribosome release, resulting in a library of polypeptides with their associated RNA still attached. Hereinafter, this and related methods are collectively referred to as “ribosome display.” Other methods employ peptides linked to RNA; for example, PROfusion technology, Phylos, Inc. See, for example, Roberts & Szostak (1997), Proc. Natl. Acad. Sci. USA, 94: 12297-303. Hereinafter, this and related methods are collectively referred to as “RNA-peptide screening.” Chemically derived peptide libraries have been developed in which peptides are immobilized on stable, non-biological materials, such as polyethylene rods or solvent-premeable resins. Another chemically derived peptide library uses photolithography to scan peptides immobilized on glass slides. Hereinafter, these and related methods are collectively referred to as “chemical-peptide screening.” Chemical-peptide screening may be advantageous in that it allows use of D-amino acids and other unnatural analogues, as well as non-peptide elements. Both biological and chemical methods are reviewed in Wells & Lowman (1992), Curr. Opin. Biotechnol, 3: 355-62. Conceptually, one may discover peptide mimetics of any protein using phage display, RNA-peptide screening, and the other methods mentioned above.
By using the phage display peptide library technique, small peptide molecules that act as agonists of the c-mpl receptor were discovered (Cwirla, S. E. et al., Science 276:1696-1699 (1997)). In such a study, random small peptide sequences displayed as fusions to the coat proteins of filamentous phage were affinity-eluted against the antibody-immobilized extracellular domain of c-mpl and the retained phages were enriched for a second round of affinity purification. This binding selection and repropagation process was repeated many times to enrich the pool of tighter binders. As a result, two families of c-mpl-binding peptides, unrelated to each other in their sequences, were first identified. Mutagenesis libraries were then created to further optimize the best binders, which finally led to the isolation of a very active peptide with an IC50=2 nM and an EC50=400 nM (Cwirla, S. E. et al., Science 276:1696-1699 (1997)). This 14-residue TPO mimetic peptide has no apparent sequence homology to TPO or MGDF. The structure of this particular TPO mimetic peptide (TMP)compound is as follows:
Ile Glu Gly Pro Thr Leu Arg Gln Trp Leu Ala Ala Arg Ala(SEQ ID NO:1)or, IEGPTLRQWLAARAusing single letter amino acid abbreviations.
Previously, in a similar study on EPO mimetic peptides, an EPO mimetic peptide (EMP) was discovered using the same technique (Wrighton, N. C. et al., Science, 273:458-463 (1996)), and was found to act as a dimer in binding to the EPO receptor (EPOR). The (ligand)2/(receptor)2 complex thus formed had a C2 symmetry according to X-ray crystallographic data (Livnah, O. et al., Science 273:464-471 (1996)). Based on this structural information, a covalently linked dimer of EMP in which the C-termini of two EMP monomers were crosslinked with a flexible spacer was designed and found to have greatly enhanced binding as well as in vitro/in vivo bioactivity (Wrighton, N. C., et al., Nature Biotechnology 15:1261-1265 (1997)).
A similar C-terminal dimerization strategy was applied to the TPO mimetic peptide (TMP). (Cwirla, S. E. et al., Science 276:1696-1699 (1997)). It was found that a C-terminally linked dimer (C-C link) of a particular TPO mimetic peptide had an improved binding affinity of 0.5 nM and an increased in vitro activity (EC50=0.1 nM) in cell proliferation assays (Cwirla, S. E. et al., Science 276:1696-1699 (1997)).
The availability of recombinant proteins for therapeutic use has led to advances in protein modifications in order to enhance or improve the properties of such proteins as pharmaceutical agents. Such modifications can provide enhanced protein protection and decreased degradation by reducing or eliminating proteolysis. Additional advantages include, under certain circumstances, increasing the stability, circulation time and biological activity of the therapeutic protein. A review article describing protein modifications is Francis, Focus on Growth Factors 3:4-10 (May 1992) (published by Mediscript, London, UK).
Useful modifications of protein therapeutic agents include linkage to polymers such as polyethylene glycol (PEG) and dextran. Such modifications are discussed in detail in a patent application entitled “Modified Peptides as Therapeutic Agents,” U.S. Ser. No. 09/428,082, PCT appl. no. WO 00/24782, which is hereby incorporated by reference in its entirety.
Another such modification is the use of an Fc region of an immunoglobulin molecule. Antibodies comprise two functionally independent parts; a variable domain known as “Fab” which binds an antigen, and a constant domain known as “Fc” which provides the link to effector functions such as complement or phagocytic cells. The Fc portion of an immunoglobulin has a long plasma half-life, whereas the Fab is short-lived. (Capon, et al. Nature 337, 525-531(1989)).
Therapeutic protein products have been constructed using the Fc domain to provide longer half life or to incorporate functions such as Fc receptor binding, protein A binding, complement fixation and placental transfer which all reside in the Fc proteins of immunoglobulins. (Capon, et al., Nature 337:525-531 (1989)). For example, the Fc region of an IgG1 antibody has been fused to CD30-L, a molecule which binds CD30 receptors expressed on Hodgkin's Disease tumor cells, anaplastic lymphoma cells, T-cell leukemia cells and other malignant cell types. See, U.S. Pat. No. 5,480,981. IL-10, an anti-inflammatory and antirejection agent has been fused to murine Fc 2a in order to increase the cytokine's short circulating half-life (Zheng, X. et al., The Journal of Immunology, 154: 5590-5600 (1995)). Studies have also evaluated the use of tumor necrosis factor receptor linked with the Fc protein of human IgG1 to treat patients with septic shock (Fisher, C. et al., N. Engl. J. Med., 334: 1697-1702 (1996); Van Zee, K. et al., The Journal of Immunology, 156: 2221-2230 (1996)). Fc has also been fused with CD4 receptor to produce a therapeutic protein for treatment of AIDS. See, Capon et al., Nature, 337:525-531 (1989). In addition, interleukin 2 has been fused to the Fc portion of IgG1 or IgG3 to overcome the short half life of interleukin 2 and its systemic toxicity. See, Harvill et al., Immunotechnology, 1: 95-105 (1995).
Published PCT Application No. WO 00/24770 discloses specific thrombopoietic compounds, generally peptides, having a tandem (i.e., N- to C-terminus) orientation and tandem peptide dimers attached at the N-terminus thereof to a carrier molecule, such as a linear polymer, an oligosaccharide or an Fc group.
There remains a need to provide additional compounds having a superior biological activity of stimulating the production of platelets (thrombopoietic activity) and/or platelet precursor cells, especially megakaryocytes (megakaryopoietic activity). There also remains a need to provide compounds that exhibit thrombopoietic activity and that also possess superior therapeutic qualities, such as a long half-life. Such compounds will exhibit advantageous properties relating to production, isolation, purification, biological activity, stability and circulation time. The present invention provides new compounds having such activity(ies), and related aspects.